fix: Fix crash when loading MAM for a contact with empty db

I think this bug happens when a user scrolls up a window, triggering
iq_mam_request_older() to load older history, in the case when the
database is empty for that contact.

The initial MAM fetch is triggered when the window is opened which
should get the latest history.

Fixes: https://github.com/profanity-im/profanity/issues/2142
Signed-off-by: Michael Vetter <jubalh@iodoru.org>

src/xmpp/iq.c

@@ -2636,10 +2636,13 @@ iq_mam_request_older(ProfChatWin* win)
     auto_gchar gchar* enddate = NULL;
 
     // If first message found
-    if (first_msg->timestamp) {
+    if (first_msg && first_msg->timestamp) {
         firstid = first_msg->stanzaid;
         enddate = g_date_time_format(first_msg->timestamp, mam_timestamp_format_string);
     } else {
+        if (first_msg) {
+            message_free(first_msg);
+        }
         return;
     }